Field of the Invention
The present invention relates to a method to lower the work function (WF) of a carbon coated LaB6 (LaB6) cathode during the manufacture thereof so that cathodes of high brightness and beam current density are provided.
Background of the Invention
Single crystal LaB6, or lanthanum hexaboride, cathodes (commonly referenced as LaB6 cathode) are used as the electron source in various electron-beam tools including lithographic tools, scanning electron microscopes (SEMs), and transmission electron microscopes (TEMs). Throughout the following description the formula LaB6 will be used to describe the material lanthanum hexaboride and the cathode structure described according to conventional practice as LaB6. A typical LaB6 cathode emitter is tapered, or cone-shaped, with a specified size, cone angle, and tip, or truncation. The tip (truncation) may be flat or spherical with a diameter ranging from 5 to 100 μm, and a cone angle may be from 60 to 110 degrees, depending on the application. A schematic drawing of a cathode showing the tip and cone is indicated in FIG. 1. The tip typically represents a (100) crystalline plane. In order to preserve the integrity of the cone shape and the tip and thus extend the useful lifetime of the cathode as a thermionic emitter, a carbon coating is applied to at least the cathode cone surface. Carbon coated cathodes are described in U.S. Pat. No. 7,176,610 and U.S. Pat. No. 9,165,737, the disclosures of which are incorporated herein by reference in their entireties.
Lanthanum hexaboride (LaB6) is a refractory crystalline material that has a melting point of 2210° C. It is purple as a crystalline material and is insoluble in water and hydrochloric acid. However, LaB6 is hygroscopic and conventionally during manufacture and storage the LaB6 cathodes are kept in inert environments which are moisture-free.
LaB6 has cubic crystalline structure and conventionally, commercial cathodes are made in such a way that the flat non-carbon coated tip represents a (100) crystalline plane, which is considered most stable and having relatively low Work Function (WF). The performance of the LaB6 cathode in terms of electron angular intensity and brightness is related to the morphological purity of the tip with regard to the (100) crystal plane. At cathode operating temperatures, LaB6 evaporates with a rate that depends on temperature and vacuum pressure. A typical evaporation loss rate may be as much as approximately 1 micron/100 hours of operation. Generally, the higher the operating temperature, the faster the evaporation rate and the effect of this evaporation is the eventual loss of tip structure wherein the (100) plane is no longer exposed, such that the cathode optics and emission are adversely affected and thus limiting the useful operational lifetime of the cathode. Therefore, it is advantageous to operate the cathode at a lowest temperature possible which still provides the emission current density required for the task, i.e. to have a LaB6 cathode with lowest WF.
The cathode emitter is generally shaped and cut in form a single crystal obtained by methods known to those of skill in the art. The tip may be obtained by a mechanical cutting operation which results in a degree of surface roughness which may be polished during the manufacture of the commercial cathode. However, even fine polishing leaves a so-called damaged crystalline sub-surface layer as thin as a fraction of a micron and electrons are emitted from this layer. The WF and brightness of the cathode is directly related to the quality of the tip.
Thus, there is a need for a manufacturing method to produce carbon coated LaB6 cathodes having tips of high morphological purity such that higher cathode brightness at a set temperature is obtained or such that an operational set brightness can be obtained at a lower working temperature. In this way, the operational lifetime of the cathode may be increased. The object of the present invention is to provide a method of cathode manufacture which enhances the cathode tip morphological integrity, i.e., eliminates the damaged sub-surface layer described previously and thus provides a cathode of low WF, extended lifetime and/or higher brightness at a given operational temperature.